Secondary radar responders

Abstract

The invention relates to secondary radar responders which provide a reply in response to a radar signal received from a primary radar transmitter. The frequency of the received radar signal is monitored by the responder and the frequency of the response is brought into agreement therewith so that the response can be received by the primary radar transmitter at its transmission frequency. The invention is particularly applicable to marine radar systems and the responder may form part of a racon or transponder.

I claim: 
     
       1. A secondary radar responder including means for receiving an interrogation pulsed radar signal, a transmitter for transmitting a pulsed response thereto in response to a received single pulse of said interrogation signal, means for monitoring the difference in frequency between said received single pulse and said transmitted response, and means for modifying the frequency of the transmitted response during transmission so as to reduce the difference, wherein said monitoring means comprise storage means for storing a signal representative of the frequency of said single pulse of said interrogation signal, and means for comparing a signal representative of the frequency of the transmitted response with the stored signal whereby the result of said comparison is used to modify the frequency of the transmitted response. 
     
     
       2. A responder as claimed in claim 1 wherein said modifying means comprise means for storing the value of the frequency of the immediately preceding response until the next following pulse of the interrogation signal is received, to enable the next response to be transmitted initially at the same frequency as the preceding response. 
     
     
       3. A responder as claimed in claim 1 wherein said monitoring means comprise means for determining whether the duration of the single pulse of the interrogation signal exceeds a predetermined period, whereby if said predetermined period is exceeded the signal which is stored in said storage means is representative of the average frequency of said single pulse present for a selected portion of its duration. 
     
     
       4. A responder as claimed in claim 1 and wherein said storage means is provided with a gain which is greater than unity, so as to minimize the residual difference between the frequency of the transmitted response and the frequency of the interrogation signal. 
     
     
       5. A responder as claimed in claim 1 wherein said modifying means comprise a frequency control circuit for generating a control signal which determines the frequency of the transmitted response in dependence on said comparison, and means for holding the value of said control signal constant unless said transmitter is actually transmitting, whereby the frequency of transmission remains constant between adjacent pulses of the same pulsed response, but is allowed to change during the transmission of a pulse. 
     
     
       6. A responder as claimed in claim 1 and wherein said comparing means comprise a threshold circuit connected for permitting the stored signal which is representative of the frequency of interrogation to be used for comparison only if the energy of the received interrogation signal exceeds a threshold value. 
     
     
       7. A responder as claimed in claim 6 and wherein the storage means comprises two elements, said stored signal being held temporarily in one of said elements until it has been determined whether the energy of the received interrogation signal exceeds the threshold value, the stored signal being transferred to the other of said elements to enable the comparison to take place only if said threshold value is exceeded. 
     
     
       8. A responder as claimed in claim 7 and wherein said storage means further comprises control means responsive to the presence of a signal indicative of a received interrogation signal whose energy exceeds said threshold for transferring the stored signal to the other of said elements of the storage means to initiate the comparison. 
     
     
       9. A responder as claimed in claim 6 wherein said receiving means comprise a code generator for determining the nature of said pulsed response, the response being transmitted in response to the reception of an interrogation signal whose energy exceeds said threshold value. 
     
     
       10. A responder as claimed in claim 9 and wherein the code generator comprises a store, from which binary bits are read out in a sequence which determines the nature of the pulsed response. 
     
     
       11. A responder as claimed in claim 10 and wherein the code generator comprises a serial shift register containing a binary sequence of stored data, and means are provided for clocking out the data of the shift register to key a transmitter oscillator on and off in accordance with the binary sequence. 
     
     
       12. A responder as claimed in claim 1 and wherein said modifying means comprise means for establishing substantial equality between the frequency of said received single pulse and the frequency of the transmitted pulsed response during the time of transmission of the pulsed response. 
     
     
       13. A secondary radar responder including means for receiving an interrogation pulsed radar signal, a transmitter for transmitting a pulsed response thereto in response to a received single pulse of said interrogation signal, means for monitoring the difference in frequency between said received single pulse and said transmitted response, and means for modifying the frequency of the transmitted response during transmission so as to reduce the difference, said responder further including a common antenna for receiving the interrogation signal and for transmitting the pulsed response, and an antenna feed having means for diverting a proportion of the energy generated by the responder transmitter to the responder receiver, so as to enable the frequency of the transmission to be compared with the frequency of the interrogation and any difference reduced. 
     
     
       14. A responder as claimed in claim 13 and wherein said means for diverting comprises a waveguide mis-match arranged to reflect a proportion of the transmitted energy. 
     
     
       15. A secondary radar responder including means for receiving an interrogation pulsed radar signal, a transmitter for transmitting a pulsed response thereto in response to a received single pulse of said interrogation signal, means for monitoring the difference in frequency between said received single pulse and said transmitted response, and means for modifying the frequency of the transmitted response during transmission so as to reduce the difference, said responder further including timing means for causing said responder to respond to an interrogation frequency of a received interrogation signal only during periodically recurring time periods. 
     
     
       16. A responder as claimed in claim 15 and wherein said timing means include means for maintaining said responder quiet apart from said periodically recurring time periods. 
     
     
       17. A responder as claimed in claim 15 and further including signal generating means for causing said responder to transmit at a predetermined frequency or frequencies except during said periodically recurring time periods. 
     
     
       18. A secondary radar responder including means for receiving an interrogation pulsed radar signal, a transmitter for transmitting a pulsed response thereto in response to a received single pulse of said interrogation signal, means for monitoring the difference in frequency between said received single pulse and said transmitted response, and means for modifying the frequency of the transmitted response during transmission so as to reduce the difference, said responder further including sub-band switching means connected for operating said responder in successive ones of a plurality of sub-bands which together constitute the total frequency band over which the respondes can accept interrogation signals.